1. Technical Field This invention relates generally to radio controlled models, and more particularly to an electronic speed controller for an RC model car.
2. Background Information
A typical RC model car race may have ten or more operators simultaneously racing 1/10 scale RC model cars around a carefully groomed 450 foot track at lap speeds in the 16-second range. But even with a model car fully outfitted and finely tuned, the winner may only edge out the runnerups by a few seconds or less. That, of course, encourages attention to details of RC model car construction and operation in preparation for the next race, and so such competitive activity makes each of those details important.
One detail concerns overall efficiency, especially efficient use of battery power, and RC racing enthusiasts go to great lengths to achieve a one or two percent improvement by such things as fine tuning gear ratios, testing and selecting battery cells, changing armature windings, and so forth. To better visualize the problem, consider a typical RC model car having a DC motor for driving the wheels and a nicad battery for supplying power to the motor. The operator varies the amount of voltage supplied to the motor, and thereby the speed of the model car, by remote control using a handheld transmitter. For that purpose, the transmitter often includes a pistol grip with a spring loaded trigger that the operator squeezes to increase speed and releases to decrease speed. But that arrangement can be a problem in high speed competition because it is so easy under stress to squeeze the trigger too much. Then excess trigger travel results in excess voltage and current and therefore excess motor torque that can overcome traction, spin the wheels, and waste battery power beyond good RC racing practice which dictates using maximum power to the extent possible without running out of power before the end of the race.
To overcome that problem, some RC model cars include an adjustable current limiter because motor torque is related to current. It is usually part of a miniature, lightweight, onboard, RC speed controller that operates conventionally in many respects to control the motor according to signals received by an onboard receiver from the handheld transmitter. The operator adjusts the current limiter to a selected level or setpoint by manipulating a miniature potentiometer and that limits the current to that setpoint regardless of how far the trigger travels. In order to determine the desired setpoint, however, the operator guesses the proper setting. Then, after operating the model car at that setting, the operator readjusts the potentiometer and operates the model car again, repeating the process until overly wasteful wheel spinning and current drainage do not occur.
But such an empirical determination may be somewhat inconvenient and time consuming. In addition, it can result in a setpoint that is not conveniently repeated, the operator basically having to guess the desired setting of a miniature potentiometer each time the setpoint is changed. Furthermore, the operator may prefer to determine the desired setpoint just before a race in order to account for constantly varying track conditions and projected racing time, but doing so may mean using precious battery power. Moreover, there may be insufficient time before the race for enough trial runs. So some better way of adjusting the setpoint is needed.